This invention relates to a process for electrostatically painting polymers. This invention particularly relates to electrostatically painting polymers having urethane or urea groups.
It is well known that most painting methods are unlikely to be 100 percent efficient. For example, when paint is applied to an object as a spray, some fraction of the paint directed at the object may not be deposited on the object. In even more inefficient circumstances, the paint and the object can acquire a static charge of the same polarity resulting in the paint being partially repelled from the object. Another painting inefficiency commonly observed is that the paint layer on a painted object can be of inconsistent thickness. Yet another painting inefficiency commonly observed, particularly in spray painting objects having complex shapes, is that paint can tend to travel from a spraying apparatus to the object to be painted in a relatively straight line and may not cover surfaces not directly accessible to the spraying apparatus.
In painting applications requiring a very high quality paint finish, such as in the painting of automobile body parts, it is generally desirable to completely and evenly coat an object to be painted with a minimum expenditure of paint. This is desirable for several reasons. First, paint for such applications is often very expensive. Therefore, reducing paint consumption offers the immediate benefit of reduced paint cost. Second, paint which is not deposited upon the object to be painted can be lost to the environment and this loss can be environmentally undesirable. Therefore, reducing the quantity of paint lost to the environment in a painting process reduces the costs of disposing of the lost paint solids and reduces emissions of paint solvents. Third, while the layer of paint being slightly too thick on an object can sometimes be tolerated, the paint layer being too thin more often cannot. Besides improving the appearance of an object, modern paints often play a vital role in protecting a painted object from its environment. For example, paint can protect a metal object from corrosion or protect a plastic object from degradation by ultraviolet radiation. Therefore, failures of painted objects due to areas of too thin paint on the objects can be avoided by applying paint to objects wherein the layer of paint is of a consistent and sufficient thickness.
In order to minimize problems with inefficient painting, it is common practice in painting some materials to apply paint electrostatically. In electrostatic painting, a static electric potential is generated between paint and an object to be painted causing the paint to be attracted to the object. As a result of the electrostatic attraction, less paint can be lost to the environment and the paint can be more evenly applied to the object without the entire surface being directly accessible to the paint spraying apparatus.
But electrostatically painting objects is not always trouble free. In order to electrostatically paint an object, an electric charge potential must be generated between both the object to be painted and a paint to be applied to the object. If an object is either not conductive or of very low conductivity, it cannot be efficiently electrostatically charged and cannot, therefore, be efficiently electrostatically painted.
One means of electrostatically painting polymers is to first make them more conductive by preparing the polymers from formulations including conductive fillers. European Patent Application 0 363 103 to Suzuki, et al., discloses preparing a thermoplastic polymer having 2 to 50 percent by weight of a fibrous conductive filler such as carbon fibers, metallic fibers, metalized glass fibers, metal coated carbon fibers and conductive potassium titanate whiskers. The polymer is etched and then electrostatically painted. However, adding such large amounts of fibrous fillers to a polymer can adversely affect both the polymer""s physical properties and paint finish. A separate etching step can also be undesirable.
JP II 2-180960 assigned to Kanto Auto Works discloses preparing polyurethane substrates having improved conductivity. The polyurethanes of this reference are prepared with ammonium salts such as n-alkyldimethyl ammonium sulphates. One problem with such additives are that they promote conductivity only when humidified. This could be a problem in a painting application located in a region wherein the ambient humidity is not constant. Alternatively, deliberately humidifying articles prior to electrostatically painting them could be time consuming and expensive.
Another means of electrostatically painting polymers is to first make them more conductive by applying thereto conductive agents called xe2x80x9cprepcoatsxe2x80x9d.These prepcoats are conductive agents which adhere to the surface of the polymer. Such agents can include materials such as quaternary amines. One problem with such compounds is that they are often hydrophilic. In reaction injection molding (RIM) polyurethanes, adsorbed water can cause blemishes on the surfaces of the polymer. Hydrophilic coatings can aggravate the tendency of objects to adsorb water prior to painting, thus promoting the formation of blemishes and thereby be undesirable in applications requiring high quality finishes.
Yet another solution to the problem of electrostatically painting plastics is to first coat a plastic object with a conductive coating, and then to electrostatically paint the coated object. For example, U.S. Pat. No. 5,071,593 to Takahashi, et. al., discloses coating problematic plastics such as polyacetal and polyesters with a conductive agent prior to electrostatic painting. However, coating an object with a conductive agent can be inefficient. It requires a capital expenditure for additional painting equipment, additional processing time to apply the conductive agent, additional processing time to allow the conductive agent to cure and expenditures for the cost of the conductive agent.
It is known in the art that polymers, particularly reaction injection molded polyurethane/polyurea polymers, are useful materials for preparing automobile parts such as, for example, fascia and interior and exterior panels. In order to paint articles prepared from such polymers and produce a high quality paint finish, as is required in modern automobile manufacturing practices, it is known to electrostatically paint these articles after first applying a conductive primer thereto. Known processes for electrostatically painting polymer articles include at least the steps of: (1) preparing an object to be painted from a polyurethane/polyurea polymer formulation; (2) coating the object with a conductive agent (or conductive primer); (3) applying an electric charge of a first polarity to a paint; (4) applying an electric charge of second and opposite polarity to the object (or merely charging either the paint or the article relative to ground while leaving the other neutral to ground); and (5) discharging the paint from a painting apparatus onto the object.
It would be desirable in the art to efficiently electrostatically paint polymers without having to pretreat the polymer by priming or prep-coating the polymer with a conductive material. It would also be desirable in the art to more efficiently electrostatically paint a polymer which has been primed or prepcoated with a conductive substance. It would be desirable in the art to prepare a polymer with sufficient native conductivity to efficiently electrostatically paint the polymer. It would be desirable in the art that the polymer to be painted not require special treatment such as humidification. And it would also be desirable in the art that the painted polymer have a sufficiently high quality painted surface and sufficiently strong physical properties such that the polymer could be used in very demanding applications such as the manufacture of automobiles. Finally, it would be desirable in the art if objects of dissimilar composition, e.g., metal and plastic, can be painted as a unit rather than painted separately and then joined together.
In one aspect, the present invention is a process for electrostatically painting cured urea polymers and/or urethane group containing polymers comprising the steps of: (A) preparing a cured polymer from a polymer formulation including a (1) materials which include or form urea groups, urethane groups or mixtures thereof, and (2) a non-volatile metal salt conductivity including material, and (B) electrostatically painting the polymer wherein (a) the polymer can be efficiently electrostatically painted, and (b) the polymer would not be conductive but for the inclusion of the non-volatile metal salt conductivity inducing materials of the present invention.
Still another aspect of the present invention is an electrostatically painted object comprising at least two layers, a first layer being a layer of polymer prepared from a polymer formulation including (1) materials which include or form urea groups, urethane groups or mixtures thereof, and (2) a non-volatile metal salt conductivity inducing material, and a second layer, the second layer being a layer of electrostatically applied paint, wherein (a) the polymer can be efficiently electrostatically painted, and (b) the polymer would not be conductive but for the inclusion of the non-volatile metal salt conductivity inducing materials in the polymer.
The present invention is an improvement to known processes for electrostatically painting polymers. This improvement includes preparing an object to be painted from a polymer formulation including (1) materials which include or form urea and/or urethane groups and (2) a non-volatile metal salt a conductivity inducing material. The resultant polymers can be electrostatically painted without the use of a conductive primer, prep-coat and the like.
In the process of the present invention, a cured polymer is painted. For the purposes of the present invention, a cured polymer is one wherein the reaction to produce the polymer is substantially complete and the polymer is in a solid and preferably fixed shape. Even more preferably, the polymer is in a shape appropriate for painting. For example, the polymer can be in the shape of an automobile body panel, side cladding or fascia.
The conductivity inducing materials of the present invention are non-volatile metal salts. As a salt the conductivity inducing materials of the present invention will have both a cation and an anion. The cation of the salts can be a cation of any metal which forms an ionizable salt with one or more anions, including Li, Be, Na, Mg, Al, K, Ca, Ga, Ge, Cu, Zn, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Rb, Sr, In, Sn, Sb, Y, Zr, Nb, Mo Tc, Ru, Rh, Pd, Ag, Cd, Cs, Ba, Tl, Pb, Bi, Po, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Fr, Ra and the Lanthanide series of the Periodic Table of the Elements. Preferably, the cation is a cation of alkali metal (Li, Na, K, Rb, Cs, Fr), an alkaline earth metal (Ca, Ba, Sr, Ra), Co, Ni, Fe, Cu, Cd, Zn, Sn, Al or Ag; more preferably the cation is a cation of an alkaline earth or alkali metal; even more preferably the cation is a monovalent cation, especially an alkali metal cation; and most preferably the cation is a cation of Li, Na, K or mixtures thereof. The non-volatile metal salt cations of the present invention can be selected from the group consisting of cations of Li, Na, K and mixtures thereof.
The non-volatile metal salt conductivity inducing materials of the present invention are salts of certain anions with the cations described above. One group of preferred non-volatile metal salts are fluoroalkyl sulfonic acid salts. The fluoroalkyl sulfonic acid anion (fluoroalkyl sulfonate) is suitably any fluoroalkyl sulfonic acid anion compatible with a specific composition in which it is used. Preferred fluoroalkyl sulfonates have from about one to about twenty carbon atoms and are either straight chained, branched or cyclic. Fluoroalkyl sulfonates are sulfonate anions having an alkyl group having fluorine substitution, that is, fluorine atoms bonded to the carbon atoms of the alkyl groups. The alkyl groups, optionally, also have hydrogen atoms and/or other halogen atoms bonded to the carbon atoms. Preferably, at least about 25 percent, more preferably about 75 percent, (by number) of the atoms other than carbon which are bonded to carbon atoms of the fluoroalkyl groups are halogen, preferably fluorine. More preferably, the fluoroalkyl groups have complete halogen substitution. Suitable halogens include fluorine, chlorine, bromine and iodine, preferably fluorine and chlorine. Suitable perhaloalkly sulfonic acid anions include, for instance, C2H2F3SO3xe2x88x92(tresylate), C2HF4SO3xe2x88x92, C2HClF3SO3xe2x88x92, C3H2F5SO3xe2x88x92, C4C2C7SO3xe2x88x92, C5H2F9SO3xe2x88x92, C7ClF14SO3xe2x88x92, C8Cl2H2F13SO3xe2x88x92, C20ClHF39SO3xe2x88x92 and the like.
The fluoroalkyl groups are most preferably perfluoroalkyl groups. Exemplary perfluoroalkyl sulfonic acid anions include, for example CF3SO3xe2x88x92(triflate), C2F5SO3xe2x88x92, C3F7SO3xe2x88x92, C4F9SO3xe2x88x92(nonaflate), C5F11SO3xe2x88x92, C6F13SO3xe2x88x92, C7F15SO3xe2x88x92, C8F17SO3xe2x88x92, C9F19SO3xe2x88x92, C/20F41SO3xe2x88x92 and the like, isomers thereof and mixtures thereof. The salts of perfluoroalkyl sulfonates preferably have from about 1 to about 20, more preferably from about 1 to about 10, carbon atoms for reasons of availability and compatibility with polymers.
Other non-volatile metal salts can be used as conductivity inducing materials to prepare the formulations of the present invention also. The anion of such salts is recognizable by those skilled in the art by such characteristics as pi bonding, electron withdrawing groups such as halogen atoms, the possibility of resonance structures and the like. The anion is preferably a relatively large, multiatomic anion having substituents like phenyl groups, sulfur atoms, phosphorus atoms and the like that can accept and delocalize an electron charge; more preferably the anion has more than one, more preferably at least about 4, most preferably at least about 5, non-metallic atoms.
Non-metallic atoms are generally considered to be selected from the group consisting of boron, carbon, silicon, phosphorus, arsenic, oxygen, sulfur, selenium, tellarium, fluorine, chlorine, bromine, iodine and astatine. Preferred non-metallic atoms are boron, phosphorus, sulfur, fluorine, and carbon in aromatic groups; sulfur, phosphorus and carbon in aromatic groups are more preferred. The anion is preferably monovalent. The anion is more preferably the conjugate base of an inorganic acid having one or more delocalizable electrons, e.g., a fluoroalkyl sulfonate or a tetraorganoboron ion. Such anions include, for example, NO3xe2x88x92, SCNxe2x88x92, SO42xe2x88x92, HSO4xe2x88x92, SO32xe2x88x92, HSO3xe2x88x92, CNSO3xe2x88x92, XSO3xe2x88x92 wherein X is a halogen, SXSO3xe2x88x92 wherein X is a hydrogen or an anion, ClO4xe2x88x92, PO43xe2x88x92, H2PO4xe2x88x92, HPO42xe2x88x92, PO33xe2x88x92, HPO32xe2x88x92, H2PO3xe2x88x92, particularly tetraalkyl and tetraarylboron ions and non-alkyl or non-aryl substituted sulfonic ions, and the like.
For the purposes of the present invention, the term non-volatile metal salt is further defined to exclude those salts which are incompatible with or undesirable in formulations for polymers having urethane and/or urea groups. For example, the anion of a non-volatile metal salt of the present invention is not an SCN- anion because the salts of these anions can cause handling problems due to viscosity growth in polyurea formulations. SCN- anions are also known to be water extractable in some polyurethane formulations. This property can cause handling problems in some painting applications. In contrast, non-volatile metal salts having good compatibility with formulations for polymers having urethane and/or urea groups are included and are preferred. For example, tetraphenylboron and hexfluorophosphate anions are particularly preferred as conductivity inducing materials for the present invention because of their good compatibility and handling properties. Mixtures of the non-volatile metal salts of the present invention can also be used to practice the present invention. Most preferably, the non-volatile metal salts of the present invention are salts wherein the non-volatile metal salt anion is selected from the group consisting of a perfluoroalkyl sulfonate, a hexafluorophosphate anion, or mixtures thereof.
The amount of conductivity inducing material which will be included in the polyurethane/polyurea formulations of the present invention will vary with the polymer. In the practice of the present invention, sufficient conductivity enhancing material is included in the formulation to render the material sufficiently conductive to allow efficient electrostatic painting. A polymer which is comparatively non-conductive can require more conductivity inducing material than a polymer which is comparatively more conductive.
xe2x80x9cHowever, generally, the amount of conductivity inducing material added to a polymer formulation useful for preparing polyurethane/polyurea polymers to be electrostatically painted is preferably from about 0.02 percent to about 1.5 percent, more preferably from about 0.05 percent to about 1.0 percent and even more preferably from about 0.10 to about 0.75 percent by weight based on the total polymer weight.xe2x80x9d
The non-volatile metal salt conductivity inducing materials of the present invention are preferably those which, when included in a polymer formulation, result in a polymer which has physical properties substantially similar to those of otherwise identical polymers prepared without the non-volatile metal salt conductivity inducing materials. The physical properties relevant to this are those properties which determine if the polymer is useful in the application for which it is intended. For example, one such property is aesthetic appearance. If a non-volatile metal salt produces an undesirable appearance in a polymer at the minimum concentration necessary to efficiently paint the polymer, it is not a preferred conductivity inducing material of the present invention. Other physical properties often useful in determining if a polymer is useful in applications wherein electrostatic painting is often done include: flex modulus, tear strength and tensile strength. For the purpose of the present invention, two polymers have substantially similar physical properties if the values for those properties are within 15 percent, preferably within 12 percent, and most preferably within 10 percent of each other. In some cases, the non-volatile metal salts of the present invention can interact with the polymers they are incorporated into to actually improve some polymer physical properties.
The polymers of the present invention can be electrostatically painted with about the same efficiency as a steel control painted under similar conditions. The efficiency of the painting process is determined by measuring the amount of paint deposited onto the object during the electrostatic painting process. For the purposes of the present invention, the term xe2x80x9cefficiently electrostatically paintedxe2x80x9d is defined as the condition wherein about the same thickness of paint is deposited upon a polymer object as is deposited upon a steel object when electrostatically painted under the same of substantially similar conditions.
The process of the present invention can be used with polymers that would not be conductive but for the inclusion of the non-volatile metal salt conductivity inducing materials of the present invention. For the purposes of the present invention, the term xe2x80x9cconductivexe2x80x9d is defined as having sufficient electrical conductivity to be efficiently electrostatically painted. The term xe2x80x9cefficiently electrostatically paintedxe2x80x9d is used as defined in the paragraph immediately before.
The polymers of the present invention have urea groups, urethane groups and mixtures thereof. That is the polymers can be prepared from materials which include or react to form only polyurethane or polyurea groups, or the polymers of the present invention can be prepared from materials which include or react to form both polyurethane and polyurea groups. Other polymer linkages can be formed in the practice of the present invention too. For example, a polymer having polyurethane, polyurea and isocyanurate groups can be prepared.
The polymers of the present invention can also be polymer blends and polymer interpenetrating network polymers. For example, a polyurethane of the present invention can be blended with another polymer such as, for example, an acrylonitrile-butadiene-styrene polymer and then be electrostatically painted. Other blendable polymers useful with the present invention include but are not limited to nylon, polyethyl terephthalate and polyacrylate. Interpenetrating network polymers can be prepared with polymers of the present invention with materials such as epoxy resins and polycarbonate resins. The network polymers can be prepared by including one or more monomers in the formulations of the present invention such that the materials form a co-continuous or phase segregated in-situ polymer network. Preferably, the urea/urethane group containing polymers are the predominant component of multipolymer compositions of the present invention.
The polymers of the present invention can be either thermoplastic or thermoset. Polyurethanes are prepared from formulations including both a polyisocyanate and a polyalcohol. Polyureas are prepared from formulations including both a polyisocyanate and a polyamine. The polyurethane/polyurea polymers are often prepared from formulations including a polyisocyanate and both a polyalcohol and a polyamine.
In the practice of the present invention, the polyisocyanate formulation component can be advantageously selected from organic polyisocyanates, modified polyisocyanates, isocyanate-based prepolymers, and mixtures thereof. These can include aliphatic and cycloaliphatic isocyanates, but aromatic and especially multifunctional aromatic isocyanates are preferred. xe2x80x9cPreferred are: 2,4- and 2,6-toluenediisocyanate and the corresponding isomeric mixtures; MDI (4,4xe2x80x2-, 2,4xe2x80x2- and 2,2xe2x80x2- diphenylmethanediisocyanate and the corresponding isomeric mixtures); (mixtures of 4,4xe2x80x2-, 2,4xe2x80x2- and 2,2xe2x80x2-diphenylmethanediisocyanates and polyphenyl polymethylene polyisocyanates (PMDI); and mixtures of PMDI and toluene diisocyanate (TDI).xe2x80x9d Also useful with the present invention are aliphatic and cycloaliphatic isocyanate compounds such as 1,6-hexamethylenediisocyanate; 1-isocyanato-3,5,5-trimethyl-1-3-isocyanatomethyl-cyclohexane; 2,4- and 2,6-hexahydrotoluenediisocyanate, as well as the corresponding isomeric mixtures; 4,4xe2x80x2-, 2,2xe2x80x2- and 2,4xe2x80x2-dicyclohexylmethanediisocyanate, as well as the corresponding isomeric mixtures.
Also advantageously used for the polyisocyanate component are the so-called modified multifunctional isocyanates, i.e., products which are obtained through chemical reactions of the above diisocyanates and/or polyisocyanates. Exemplary are polyisocyanates containing esters, ureas, biurets, allophanates and preferably carbodiimides and/or uretonimine; isocyanurate and/or urethane group containing diisocyanates or polyisocyanates. Liquid polyisocyanates containing carbodiimide groups, uretonimine groups and/or isocyanurate rings, having isocyanate groups (NCO) contents of from 10 to 40 weight percent, more preferably from 20 to 35 weight percent, can also be used. These include, for example, polyisocyanates based on 4,4xe2x80x2-, 2,4xe2x80x2- and/or 2,2xe2x80x2-diphenylmethane diisocyanate and the corresponding isomeric mixtures, 2,4- and/or 2,6-toluenediisocyanate and the corresponding isomeric mixtures, 4,4xe2x80x2-, 2,4xe2x80x2- and 2,2xe2x80x2-diphenylmethane diisocyanate and the corresponding isomeric mixtures; mixtures of diphenylmethane diisocyanates and PMDI and mixtures of toluene diisocyanates and PMDI and/or diphenylmethane diisocyanates.
Suitable also are prepolymers having NCO contents of from 5 to 40 weight percent, more preferably from 15 to 30 weight percent. These prepolymers are prepared by reaction of the di- and/or polyisocyanates with materials including diols, triols, but also they can be prepared with multivalent active hydrogen compounds such as di- and tri-amines and di- and tri-thiols. Individual examples are aromatic polyisocyanates containing urethane groups, preferably having NCO contents of from about 5 to about 40 weight percent, more preferably about 20 to 35 weight percent, obtained by reaction of diisocyanates and/or polyisocyanates with, for example, lower molecular weight diols, triols, oxyalkylene glycols, dioxyalkylene glycols or polyoxyalkylene glycols having molecular weights up to about 800. These polyols can be employed individually or in mixtures as di- and/or polyoxyalkylene glycols. For example, diethylene glycols, dipropylene glycols, polyoxyethylene glycols, polyoxypropylene glycols and polyoxypropylenepolyoxyethylene glycols can be used.
Particularly useful in the present invention are: (i) polyisocyanates having an NCO content of from 8 to 40 weight percent containing carbodiimide groups and/or urethane groups, from 4,4xe2x80x2-diphenylmethane diisocyanate or a mixture of 4,4xe2x80x2- and 2,4xe2x80x2-diphenylmethane diisocyanates; (ii) prepolymers containing NCO groups, having an NCO content of from 20 to 35 weight percent, based on the weight of the prepolymer, prepared by the reaction of polyoxyalkylene polyols, having a functionality of preferably from 2 to 4 and a molecular weight of from about 800 to about 15,000 with 4,4xe2x80x2-diphenylmethane diisocyanate or with a mixture of 4,4xe2x80x2- and 2,4xe2x80x2-diphenylmethane diisocyanates and mixtures of (i) and (ii); and (iii) 2,4- and 2,6-toluenediisocyanate and the corresponding isomeric mixtures. PMDI in any of its forms can also be used and is preferred. In this case it preferably has an equivalent weight between about 125 and about 300, more preferably from about 130 to about 175, and an average functionality of greater than about 2. More preferred is an average functionality of from about 2.5 to about 3.5. The viscosity of the polyisocyanate component is preferably from about 25 to about 5,000 centipoise (cps) (about 0.025 to about 5 Paxc2x7s), but values from about 100 to about 1,000 cps (about 0.1 to about 1 Paxc2x7s)at 25xc2x0 C. are preferred for ease of processing. Similar viscosities are preferred where alternative polyisocyanate components are selected.
In the practice of the present invention, a polyalchohol formulation component can be advantageously selected from the following classes of compositions, alone or in admixture: (a) alkylene oxide adducts of polyhydroxy alkanes; (b) alkylene oxide adducts of non-reducing sugars and sugar derivatives; (c) alkylene oxide adducts of phosphorus and polyphosphorus acids; and (d) alkylene oxide adducts of polyphenols. Polyols of these types are referred to herein as xe2x80x9cbase polyolsxe2x80x9d.Examples of alkylene oxide adducts of polyhydroxyalkanes useful herein are adducts of ethylene glycol, propylene glycol, 1,3-dihydroxypropane, 1,4-dihydroxybutane, and 1,6-dihydroxyhexane, glycerol, 1,2,4-trihydroxybutane, 1,2,6-trihydroxyhexane, 1,1,1-trimethylolethane, 1,1,1-trimethylolpropane, pentaerythritol, polycaprolactone, xylitol, arabitol, sorbitol, mannitol, and the like. Preferred herein as alkylene oxide adducts of polyhydroxyalkanes are the ethylene oxide adducts of trihydroxyalkanes. Other useful adducts include ethylene diamine, glycerin, ammonia, 1,2,3,4-tetrahydroxy butane, fructose, and sucrose.
Also preferred are poly(oxypropylene) glycols, triols, tetrols, pentols and hexols and any of these that are capped with ethylene oxide. These polyols also include poly(oxypropyleneoxyethylene)polyols. The oxyethylene content should preferably comprise less than about 80 weight percent of the total and more preferably less than about 40 weight percent. Ethylene oxide, when used, can be incorporated in any way along the polymer chain, for example, as internal blocks, terminal blocks, or randomly distributed blocks, or any combination thereof.
Another preferred class of polyols are xe2x80x9ccopolymer polyolsxe2x80x9d, which are base polyols containing stably dispersed polymers such as acrylonitrile-styrene copolymers. Production of these copolymer polyols can be from reaction mixtures comprising a variety of other materials, including, for example, catalysts such as azobisisobutyronitrile; copolymer polyol stabilizers; and chain transfer agents such as isopropanol.
Polyisocyanate polyaddition active hydrogen containing compounds (PIPA) are also useful for preparing formulations of the present invention. PIPA compounds are typically the reaction products of TDI and triethanolamine. A method for preparing PIPA compounds can be found in, for example, U.S. Pat. No. 4,374,209, issued to Rowlands. Polyester polyols can be used for preparing the polymers of the present invention. For example, polyols prepared from caprolactone are useful. Polyols prepared from butanediol and adipic acid can also be used. Any polyester known to one skilled in the art of preparing polyurethanes and polyureas to be useful can be used with the present invention.
Low molecular weight diols and triols can also be used in preparing the polymers of the present invention. Ethylene glycol is particularly useful but other, similar compounds can also be used. Propylene glycol, diethylene glycol, are also suitable for use in the present invention.
In the practice of the present invention, the polyamine formulation component can be selected from the group including and polyamines, and amineterminated polyols. Polyamines are preferred for preparing the polyurethane/polyurea formulations of the present invention include the known low molecular isocyanate-reactive compounds such as aromatic polyamines, especially diamines, having molecular weights of less than about 800, preferably less than about 500.
Preferred amine group containing compounds include the sterically hindered aromatic diamines which contain at least one linear or branched alkyl substituent in the ortho position to the first amino group and at least~one, preferably two, linear or branched alkyl substituents containing at least one, preferably one to three carbon atoms in the ortho position to the second amino group. These aromatic diamines include 1-methyl-3,5-diethyl-2,4-diaminobenzene, 1-methyl-3,5-diethyl-2,6-diaminobenzene, 1,3,5-trimethyl-2,4-diaminobenzene, 1-methyl-5-t-butyl-2,4-diaminobenzene, 1-methyl-5-t-butyl-2,6 diaminobenzene, 1,3,5-triethyl-2,4-diaminobenzene, 1-methyl-5-t-butyl-2,4-diaminobenzene, 1-methyl-5-t-butyl-2,6-diaminobenzene, 1,3,5-triethyl-2,4-diaminobenzene, 3,5,3xe2x80x2, 5xe2x80x2-tetraethyl-4,4xe2x80x2-diaminodiphenylmethane, 3,5,3xe2x80x2, 5xe2x80x2-tetraisopropyl-4,4xe2x80x2-diaminodiphenylmethane, 3,5-diethyl-3xe2x80x2, 5xe2x80x2-diisopropyl-4,4xe2x80x2-diaminodiphenylmethane, 3,3xe2x80x2-diethyl-5,5xe2x80x2-diisopropyl-4,4xe2x80x2-diaminodiphenylmethane, 1-methyl-2,6- diamino-4-isopropyibenzene and mixtures of the above diamines. Most preferred are mixtures of 1-methyl-3,5-diethyl-2,4-diaminobenzene and 1-methyl-3,5-diethyl-2,6-diaminobenzene in a weight ratio between about 50:50 to 85:15, preferably about 65:35 to 80:20.
Unhindered aromatic polyamines can be used with the sterically hindered chain extenders and include 2,4-and/or 2,6-diaminotoluene, 2,4xe2x80x2 and/or 4,4xe2x80x2-diaminodiphenylmethane, 1,2xe2x80x2- and 1,4-phenylene diamine, naphthalene-1,5-diamine and triphenyl methane -4,4xe2x80x2, 4xe2x80x3-triamine. The difunctional and polyfunctional aromatic amine compounds may also exclusively or partly contain secondary amino groups such as 4,4xe2x80x2-di-(methylamino)-diphenylmethane or 1-methyl-2-methylamino-4-Aminobenzene. Liquid mixtures of polyphenyl polymethylene polyamines of the type obtained by condensing aniline with formaldehyde are also suitable.
Generally the nonsterically hindered aromatic diamines and polyamines are too reactive to provide sufficient processing time in preparing polymers such as RIM polyurethanes and polyureas. Accordingly, these diamines and polyamines should be used in combination with one or more of the previously mentioned sterically hindered diamines. One exception to this is the case of methylene diorthochloroaniline. This particular diamine, though not sterically hindered, is a suitable material for preparing RIM polyurethane/polyureas.
Polyurethane catalysts are also suitably used with the present invention. The catalyst is preferably incorporated in the formulation in an amount suitable to increase the rate of reaction between the isocyanate groups of the composition of the present invention and a hydroxyl-reacting species. Although a wide variety of materials is known to be useful for this purpose, the most widely used and preferred catalysts are the tertiary amine catalysts and the organotin catalysts.
Examples of the tertiary amine catalysts include, for example, triethylenediamine, N-methyl morpholine, N-ethyl morpholine, diethyl ethanolamine, N-coco morpholine, l-methyl-4-dimethylaminoethyl piperazine, 3-methoxy-N-dimethylpropylamine, N,N-diethyl-3-diethyl aminopropylamine, dimethylbenzyl amine and the like. Tertiary amine catalysts are advantageously employed in an amount from about 0.01 to about 2 percent by weight of the polyol formulation.
Examples of organotin catalysts include dimethyltin dilaurate, dibutyltin dilaurate, dioctyltin dilaurate, stannous octoate and the like. Other examples of effective catalysts include those taught in, for example, U.S. Pat. No. 2,846,408. Preferably the organotin catalyst is employed in an amount from about 0.001 to about 0.5 percent by weight of the polyol formulation.
Catalysts which promote the formation of isocyanurate groups can also be used with the present invention. Suitable catalysts for use with the present invention include such as those mentioned in Saunders and Frisch, Polyurethanes, Chemistry and Technology in xe2x80x9c1 High Polymersxe2x80x9d Vol. XVI, pp. 94-97 (1962). Such catalysts are referred to herein as trimerization catalysts. Examples of these catalysts include aliphatic and aromatic tertiary amine compounds, organometallic compounds, alkali metal salts of carboxylic acids, phenols and symmetrical triazine derivatives. Preferred trimerization catalysts are potassium salts of carboxylic acids such as potassium octoate and tertiary amines such as, for instance, 2,4,6-tris(dimethyl aminomethyl) phenol.
The process of the present invention can include an additional step wherein the polymer is formed into an article prior to painting it. RIM, as already discussed above is a preferred method of preparing an article. Injection molding of thermoplastics is also a preferred means of preparing an article. Polymer casting can also be practiced with the process of the present invention as well as blow molding, extrusion and compression molding. One advantage of the present invention is that articles so formed can be attached to metal articles and the two painted together as a unit.
In addition to the above formulations components, the polyurethane/polyurea formulations of the present invention can also include materials known to be useful in preparing polyurethane/polyurea polymers by those skilled in the art. Included in these materials are additives such as fillers, mold release agents, pigments, blowing agents, surfactants, flame retardants and the like. Specifically excluded are other conductive fillers. The polymers of the present invention are not sufficiently conductive in the absence of the non-volatile metal salt conductivity inducing materials of the present invention to be efficiently electrostatically painted.
The formulation components of the formulations of the present invention can be brought together to form a polymer in any way known to be useful to those skilled in the art of preparing polyurethane/polyurea polymers. One preferred means of forming the polyurethane/polyurea polymers of the present invention is by means of reaction injection molding (RIM). Preparing RIM polymers is well known in the art, but generally includes the steps of introducing at least two streams of mutually reactive materials through a mixer into a mold wherein the materials polymerize to produce a molded polymer article.
In addition to the advantage of being paintable without the use of a conductive primer, the polymers of the present invention can be molded with fewer defects due to flash imperfections. Small particles of polymer which remain in the mold after a molded article has been removed from the mold are known as flash. Since the polymers of the present invention are comparatively more conductive than otherwise identical polymers prepared without conductivity inducing materials, they can be less subject to a static-attraction to the mold and can be more easily removed. Therefore, the flash particles are less likely to remain behind and be an imperfection on the surface of the next article to be molded.
In the practice of the present invention, cured polymers are electrostatically painted. For the purposes of the present invention, electrostatic painting includes at least the steps of: (1) charging an object to be painted with an electrical charge, (2) charging a paint with an electrical charge of opposite polarity to that of the object or at least grounding the paint relative to the charged article and; (3) dispensing the paint from an electrostatic painting apparatus onto the object. It is also possible and even more routine, in some industries, to charge the paint and ground the object to be painted. Any means and apparatus known to be useful for electrostatic painting to those skilled in the art can be used with the process of the present invention. For example, an apparatus such as a BINKS MODEL 85* can be used to electrostatically paint the cured polymers of the present invention (*BINKS MODEL 85 is a trade designation of Binks Manufacturing Company).
While the present invention can be used with liquid paints, it is not intended that the term xe2x80x9cpaintxe2x80x9d be so narrowly construed. For purposes of the present invention, the material which can be xe2x80x9cpaintedxe2x80x9d onto an object by the method the present invention includes any material which can be electrostatically deposited onto an object. These materials include but are not limited to liquid pigment paints, liquid transparent paints (also known as clear coats), powder pigments, powder coatings, conductive coatings such as primers and prepcoats and the like. The materials can be neat or solvent born, water born or both solvent and water born. As with the case of powders, for example, the materials can also be applied as a solid. The process of electrostatic painting includes the process of electrodeposition as well wherein rather than applying the material onto the article in an aerosol, the article is dipped into the material while an electrostatic potential is maintained between the article and the material being applied thereto.
A preferred embodiment of the present invention is an electrostatically painted object comprising at least two layers, a first layer being a layer of polymer prepared from a polymer formulation including (1) materials which include or form urea groups, urethane groups or mixtures thereof, and (2) a non-volatile metal salt conductivity inducing material, and a second layer, the second layer being a layer of electrostatically applied paint, wherein (a) the polymer can be efficiently electrostatically painted, and (b) the polymer would not be conductive but for the inclusion of the non-volatile metal salt conductivity inducing materials in the polymer. While the present invention is useful for electrostatically painting articles not primed or coated with a conductive material, that is the two layers being adjacent, the present invention can also be used advantageously to prepare articles having a third layer interposed between the electrostatically applied paint and the polymer. The third layer can be a non-conductive or a conductive coating. The non-volatile metal salts of the present invention increase the bulk conductivity of polymers prepared therewith. Surface coating of a polymer with a conductive material increases surface conductivity but has little effect on bulk conductivity. It is believed that surface conductivity alone is inefficient in charging an article to be electrostatically painted. Therefore, the method of the present invention can be employed to more efficiently paint a polymer coated with a non-conductive layer or even a polymer having a preparatory conductive coating such as a conductive primer or prep-coat.
The method of the present invention can be advantageous compared to conventional methods of electrostatically painting polymers for several reasons. One of these reasons is that a very small amount of material is added to a polymer which both imparts sufficient conductivity to permit efficient electrostatic painting but does not substantially degrade polymer physical properties. The non-volatile metallic salts of the present invention can be selected such that they are compatible with the polymer formulation in which they will be included. Alternatively, the salt can be mixed with a compatibilizer. If a compatibilizer is used, it should be selected such that it does not impart undesirable properties to the polymer and that it does not degrade the physical properties of the polymer. For example, n-methyl-2-pyrrolidone can be used to compatibilize a non-volatile salt of the present invention and yet not cause a substantial degradation of polymer physical properties prepared therewith.
In addition to the other benefits of the present invention, the process of the present invention can be safer to use than conventional processes of electrostatically painting polymers. In some conventional painting processes, a conductively primed part is painted. Since there is primarily only surface conductivity on such a part, substantial energy must be used for painting. If the lead to the part becomes loose, the possibility for fire due to arcing of the lead to the part is substantial. The process of the present invention utilizes bulk conductivity which can lower the possibility for arcing and thus fires.